Prepolymer process control

In principle, both the one-shot process and semi-prepolymer processes have been used for rigid-urethane-foam manufacturing. However, the monomeric TDI-based one-shot process was used only in the initial stage of the rigid-urethane-foam industry because of the toxicity problems of TDI and difficulties in controlling reactivity due to the high NCO percent. For these reasons TDI-prepolymers, blends of TDI prepolymers and polymeric isocyanates, and 100% polymeric isocyanate are most widely used. 

Vacuum film drying. In this process a film of the liquid polyurethane prepolymer flows over a weir and runs down the side of a supporting column. The weir and face of the column down which the liquid polyurethane flows is subjected to a high vacuum which degasses the liquid polyurethane. An example is the APC vacuum falling film degasser (Automatic Process Control, California, USA). 

This method produces polyurethanes with a controlled composition. Alternatively, the polymer can be produced by the one step ( one shot ) process, where all components are mixed together at the same time. The resulting polymer has statistical composition (random distribution of polyol and chain extender units in the chain), which depends on the relative reactivity of different diol components. The properties would differ somewhat from those of the polymers made by the prepolymer process. Soft segment concentration is controlled by the chain extender/polyol ratio. The following formula (48) can be used to calculate chain extender (CE)/polyol (POL) molar ratio (r) for the desired soft segment concentration, SSC ... 

In the first step of the polymerization process, a prepolymer is prepared as a slurry in water. Excess diamine is added to control the degree of polymerization, eg, degree of polymerization = 6-14 (158). This prepolymerization step is conducted at approximately 200°C under autogenous pressure for less than 90 min. 

Continuous polymerization processes for PA-6,6 have been reported for over 30 years.5,6,28 Prepolymerization in tubular (Fig. 3.21) or baffled reactors is particularly well suited to continuous polymerization. The polymerization of prepolymers to high-molecular-weight materials in a continuous process is more difficult to control as small differences is molecular weights result in large differences in viscosities. Viscosity differences result in different hold-up times in die reactor and thus nonhomogeneous products. 

The process begins in a prepolymerizer, which is a water-jacketed reactor with a mixer in it. See Figure 23—12.) The styrene is partially polymerized by adding the peroxide initiator and heating to 240—250°F for about four hours. About 30% of the styrene polymerizes and the reactor contents become syrupy goo. Thats about as far as the prepolymer step can go—30% conversion— because the mixing and heat transfer gets very inefficient as the goo gets thicker, and the polymerization becomes hard to control. 

First and most importantly, real-time NIR monitoring enabled real-time control of the process. For a given product, the molecular weight and end-group balance in the prepolymer exiting the front end or melt part of the process must be controlled at specified levels in order for the back end or solid-phase part of the process to successfully produce the intended polymer composition. In addition, the variability in prepolymer composition must be controlled with very tight tolerances to keep the variation in final product composition within specification limits. Since the process dynamics in the front end were more rapid than those in conventional PET processes, the conventional analytical approach involving off-line analysis of samples obtained every 2-A hours was not sufficient to achieve the desired product quality. 

The resistance to fluid flow is a measure of the physical structure of the foam. In order to control the flow through a foam, ceU size, degree of reticulation, density, and other physical factors must be controlled. The control of these physical factors, however, is achieved through the chemistry and the process by which the foam is made. The strength of the bulk polymer is measured by the tensile test described above, but it is clear that the tensile strengths of the individual bars and struts that form the boundaries of an individual cell determine, in part, the qualities of the cells that develop. A highly branched or cross-linked polymer molecule will possess certain tensile and elongation properties that define the cells. The process is also a critical part of the fluid flow formula, mostly due to kinetic factors. As discussed above, the addition of a polyol and/or water to a prepolymer initiates reactions that produce CO2 and cause a mass to polymerize. The juxtaposition of these two reactions defines the quality of the foam produced. Temperature is the primary factor that controls these reactions. Another factor is the emulsification of the prepolymer or isocyanate phase with the polyol or water. 

We have described a process by which small quantities of foam can be made by the prepolymer method. A number of methods are available to bring the variables involved in prepolymer making under control. We assume that the starting point for such processes is the acquisition of commercial isocyanates, polyols, and additives. Other than for audit purposes, we will assume they arrive with certification that they are of the so-called urethane grade. In the case of polyols, this typically means they contain less than 0.01% water and have good color. Free acids and low metal and chloride contents are important considerations for isocyanates. Manufacturers are well aware of the problems that will arise if these contaminants are not controlled and the materials cannot be used with confidence. 

Special rules apply to the world of hydrophilic polyurethanes. These alternate rules are based on the fact that hydrophilic polyurethanes can and should be processed in water. Rather than emulsifying a prepolymer with a polyol, as would be done with a hydrophobic polyurethane, hydrophilics are mixed with water. While the properties of the foam are governed loosely by the guidelines described above, one has more flexibility and control of the formulation and process by which the polyurethane is made. For example, the water can serve as a heat sink to closely control the temperature of the foam the water controls the rate of reaction. 

All hydrophilics are currently processed by the prepolymer method. The emulsification of the prepolymer and water are the primary determinants of cell size. The water also serves as a heat sink to moderate the temperature of the reaction. By adjusting the temperatures of the prepolymer and the water, one can control the kinetics described above. The mass of the water limits the destructive exotherm. 

While absorption is a valuable quality, more often than not absorption capacity is used to carry solutes into the matrix of a foam. If this is done after the foam is produced, the process is referred to as imbibing. It is sometimes advisable to add the solute to the water phase used to produce the polyuretliane. As long as the solute is not reactive with the prepolymer, the solute is deeply imbedded into the foam matrix. The imbibing process has a tendency to place the solute closer to the surface. The purpose is usually delivery in a controlled manner. The simplest example is to imbibe a liquid soap into a foam that is then dried. The soap diffuses out when the foam is wet. 

The process of producing a prepolymer is one of adding two or more liquids together under controlled conditions to obtain a prepolymer. To maintain... 

The preparation of prepolymers [111] or macromers with functional end groups, so called telechelic polymers, is another approach to structurally unconventional architecture. The functional end groups are introduced either by functional initiation or end-capping of living polymers, or by a combination of the two. In this way, monomers that are not able to copolymerize can be incorporated in a copolymer. Telechelic prepolymers can be linked together using chain extenders such as diisocyanates [112]. In this process, it is essential that the structure and end groups of the prepolymers can be quantitatively and qualitatively controlled [113]. 

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